It can and does happen. For example, sex offenders in Oregon, can be restricted from owning or using ANY device that is capable of accessing the internet for the length of their post prison supervision or parole.
This can be and is verified by the parole/PPS officer that will inspect the address requested as a release address and if there are devices that violate these restrictions they must be removed from the household BEFORE the offender is allowed to live there.
This law is easy to enforce as a condition of probation, parole or post prison supervision and the idea that its "to big a job" to monitor whether a sex offender is using myspace or facebook is just a whiny cop out for those jurisdictions that are unwilling to simply ban the ownership or use of these devices by sex offenders.
And before anyone cries "unconstitutional government restrictions" I would remind them that the banning of felons from possessing or owning "firearms" has been upheld time and again.
These offenders use a computer like a weapon, preying on children online and I for one, have NO problem denying ANY sex offender access to the internet for the rest of their miserable lives.
I have a better idea. death penalty.